1. Field of the Invention
The invention relates to transceivers providing full-duplex communication and in particular to a hybrid circuit for a full-duplex transceiver.
2. Description of Related Art
Conventional modems operating in accordance with an Asymmetrical Digital Subscriber Line (ADSL) standard incorporate an echo canceling hybrid circuit for combining a transmit signal with a receive signal in an ADSL transceiver to enable bidirectional data communication through a two-wire transmission line, such as, for example, an unshielded twist wire pair. The hybrid circuit cancels the echo signal caused by a portion of the transmit signal appearing in the receive signal as noise.
FIG. 1 shows a typical prior art transceiver 10 for forwarding an input signal TX as a transmitted signal TX′ to a remote transceiver (not shown) via a transmission line 12, and for generating an output signal RX in response to a received signal RX′ arriving on line 12 from the remote transceiver. Transceiver 10 includes an amplifier 11, for amplifying the input TX signal, a summing amplifier 16 for generating the output RX signal, and a hybrid circuit 18 for coupling transmission line 12 to the output of amplifier 11 and the input of amplifier 16. Hybrid circuit 18 includes an impedance element Z1 and a transformer 14 coupling the output of amplifier 11 to line 12. Transformer 14 also couples a non-inverting input of amplifier 16 to line 12. Both the received and transmitted signals RX′ and TX′ on line 12 appear as components of a signal VIN at the non-inverting input of summing amplifier 16. Hybrid circuit 18 also includes a voltage divider network comprising impedance elements Z2 and Z3 to provide an offset signal VOFF at an inverting input of amplifier 16 to offset the transmitted signal component of the VIN signal. The VOFF signal will cancel the echo of transmitted signal TX′ in received signal RX′ when Z1/ZL=Z2/Z3, where ZL is the impedance of transmission line 12.
A modem meeting the ADSL standard uses a frequency division duplex (FDD) scheme wherein the transmitted and received signals TX′ and RX′ on line 12 occupy different frequency bands. For example, for customer premise equipment (CPE) ADSL modem, the transmitted signal occupies the 30 KHz-138 KHz frequency band (the “upstream band”) while the received signal occupies the 138 KHz-1.1 MHz frequency band (the “downstream band”). Although the line driver of a CPE modem delivers the transmitted signal over the upstream band, the transmitted signal TX′ inevitably introduces some level of noise and distortion extending above 138 KHz and lying in the downstream band of the received signal RX′. Hybrid circuit 18 does not completely cancel the residual echo of transmit signal TX′ in output signal RX. One portion of this residual echo resides in the transmitted signal's upstream band, and a remaining portion of the residual resides in the received signal's downstream band. Since the residual echo in the receiver band is indistinguishable from the received signal RX′, it can significantly degrade receiver performance.
A filter (a high pass filter in the case of a CPE transceiver) can separate the residual echo in the transmitted band, but if the residual echo is too strong, it can saturate the receiver, particularly its low noise amplifier. Therefore, it is important for a transceiver to include a hybrid circuit providing good echo cancellation to minimize the residual echo in both the upstream and downstream bands. It is not difficult to design a hybrid circuit to provide sufficient echo cancellation in both the transmitter band and the receiver band when line 12 is an ideal twist-pair transmission line. However, a twisted-pair transmission line will often include bridged taps (open-end transmission line stubs) that cause the transmission line impedance to vary significantly with frequency, and the lengths and locations of the bridged taps strongly affect the relationship between transmission line impedance and signal frequency. Designers therefore find it difficult to design a hybrid circuit that can provide good echo cancellation in both the transmitter band and the receiver band that can accommodate a wide variety of transmission line frequency response characteristics. A common solution is to make the impedance element Z3 of hybrid circuit 18 adjustable using several discrete switches to switch resistors and/or capacitors in or out of element Z3 to synthesize an impedance providing a good match to the line impedance.
The following patents describe hybrid circuit topologies addressing transmission line impedance matching problems associated with bridged taps:
U.S. Patent Publication 2003/0147526, filed Aug. 7, 2003 by Oswal et al.
U.S. Patent Publication 2003/0123650, filed Jul. 3, 2003 by Oygang.
U.S. Patent Publication 2003/0169875, filed Sep. 11, 2003 by Lee et al.
U.S. Patent Publication 2003/0169806, filed Sep. 11, 2003 by Warke.
U.S. Pat. No. 6,208,732 issued Mar. 27, 2001 to Muschytz et al.
FIG. 2 illustrates another prior art transceiver 20. An input signal TX drives a differential amplifier 22 driving a transmission line 26 via a hybrid circuit 21 to produce the transmitted signal TX′. Hybrid circuit 21 also couples transmission line 26 to the input of a differential amplifier 28 having feedback provided by impedance elements Z3. Amplifier 28 amplifies the received signal RX′ signal arriving from line 26 to produce an output signal RX.
Hybrid circuit 21 includes a transformer 24 having a primary winding coupled to line 26, a secondary winding coupled to the output of amplifier 22 through impedance elements Z1, and another secondary winding coupled to the input of amplifier 28 through impedance elements Z2. Impedance elements Z1 match the nominal impedance of transmission line 26. For example, if the nominal line impedance is 100 Ohms, each impedance element Z1 should be 50 Ohms, assuming the turn ratio of the transformer windings is 1:1 between the transmit (TX) side and the line side of the transformer. Impedance elements Z2 present an impedance much higher than elements Z1 so that the impedance looking into hybrid circuit 21 from line 26 substantially matches the nominal line impedance 2·Z1. In inverting configuration, amplifier 28 acts as a low noise amplifier providing a gain of −Z3/Z2 to the signal arriving on line 26. A passive circuit H1 including series impedance elements H1Z residing between the output of line driver 22 and the input of amplifier 28. The reverse polarity of the differential signal flowing through the passive circuit H1 cancels the echo. If the nominal line impedance is, for example, 100 Ohms and the primary and secondaries of transformer 24 have identical numbers of turns, impedance elements Z1 can be 50 Ohm resistors and impedance element Z2 can be, for example, 1 KOhm. If the output of amplifier 22 is 1V, the voltage coupled to both the line side and the RX side of the transformer 24 will be approximately 0.5V. The echo at the output of amplifier 28 due to the coupling between the transformer secondaries will be 0.5V*Z3/1 KOhm. The signal voltage at the output of amplifier 28 due to the path through passive circuit H1 will be 1V*Z3/H1Z. To provide a good echo cancellation, H1Z should satisfy the following:0.5V*Z3/1 KOhm=1V*Z3/H1Z. This implies H1Z should be 2 KOhm. Since the line impedance for an ideal twist-pair transmission is a function of frequency, a simple series resistor implementation of H1 will not provide sufficient echo cancellation. However, a simple 2nd order RC network can provide sufficient echo cancellation. In general, if the line driver output is 1V and the line impedance is ZL, then the echo at the output of amplifier 28 due to the coupling between TX side and RX side of the transformer is approximately 1V*ZL/(2*Z1+ZL)*Z3/Z2. The signal at the output of amplifier 28 due to the path through circuit H1 will be 1V*Z3/H1Z. To achieve adequate echo cancellation, the single-end impedance H1Z of passive circuit H1 should satisfy1V*ZL/(2*Z1+ZL)*Z3/Z2=1V*Z3/H1Z. This impliesH1Z=Z2*(1+2*Z1/ZL).
The design of H1 becomes more difficult when transmission line 26 includes bridged taps and is therefore not an ideal twisted pair. Since the line impedance ZL is highly frequency dependent for a line 26 having bridged taps, it is not possible to achieve good echo cancellation using simple passive RC networks when line 26 has bridged taps. For a given loop length and bridged tap configuration, it is possible to replace the passive RC network H1 with a high order passive RLC network approximating the impedance Z2*(1+2*Z1/ZL). However, since inductors are expensive compared with other discrete circuit elements it can be preferable to use an active filter in lieu of an RLC network.
FIG. 3 illustrates a prior art transceiver 30 that is similar to transceiver 20 of FIG. 2 except that hybrid circuit 21 of transceiver 30 employs a 2nd order active filter circuit HA in place of passive circuit H1 of FIG. 2. FIG. 4 illustrates a prior art 2nd order active filter HA of the FIG. 3. In an integrated circuit, an active filter can be a low cost alternative to an expensive discrete RLC network because it can approximate any impedance, provided it does not contain right half plane poles that cause instability. However, an active filter introduces noise into a hybrid circuit, and the noise increases with the order of the filter. When the noise introduced by active filter HA is significant, it undermines the function of the low noise amplifier 28.
The unshielded twisted air transmission line 26 attenuates the received signal originating from the remote transmitter with, for example, a 40 dB loss and the receiver usually requires a few stages of amplification after amplifier 28 to boost the signal level to adequately compensate for the attenuation so that the signal is suitable for signal processing. The signal level sequentially increases from the first stage to the last stage, but each stage of amplification introduces added noise. The added noise is particularly detrimental in the earliest stage(s), since the signal levels are lowest. Therefore, it is important to use low noise amplifiers in the earliest stage(s) of amplification, however the circuit noise added by the introduction of active filter HA in hybrid circuit 30 can undermine the low noise function of amplifier 28. Thus to preserve signal integrity, active filter HA should be designed for low noise, but a low noise active filter is difficult to design, particularly when the order of the filter is high.
What is needed is a hybrid circuit architecture for use in connection with transmission lines having a variety of frequency-dependant impedances that does not require expensive RLC networks and that employs only easily designed, low noise, low order, active filters.